Tuesday, December 13, 2011

Ambient light sensor on the roof

Installed an ambient light level sensor on the roof. Voltage from the simple voltage divider made up of an LDR and a fixed 5% resistor (750Kohm in this case) is digitized by the analog-to-digital converter of the PIC12F1822 (16 readings are taken successively--takes around 1ms--and averaged) and is sent out through the its serial port at 19.2kbps every 50ms. Communications is simplex--this transmitter merely sends while receivers in remote locations can only receive data. Serial data from the MCU is fed to the SN75176B transceiver which then converts the data stream to an RS-485 compliant signal which is then sent down a pair of unshielded twisted pair cable, for use by any circuit requiring the data. Checksum is sent with every data packet for data integrity verification.

A pair of wires in the UTP cable provides filtered (but unregulated) power to the board. Because the input voltage is somewhere around 8 to 8.5V and may drop below 8 if loads elsewhere (various circuits use the same power supply) increase, I used a 1N5817 Schottky diode instead of a 1N400x for polarity reversal protection. That halves the forward voltage drop across the diode to just 0.3V and thus increases the headroom for the 78L05, ensuring it's able to maintain voltage regulation. There are separate power and ground tracks/traces for the analog and digital sections to minimize digital noise from contaminating the sensor section. The analog section has a 1uF tantalum filter cap and the MCP6232 op amp has the mandatory 0.1uF decoupling cap. You might notice the 1/4-watt resistor adjacent to the LDR. Reason is the dang supplier doesn't have the complete range of values for 1/8-watt resistors.

Image below shows the board in situ with the translucent plastic cover yet to be snapped on. Screws on the board act as standoffs to keep the soldered side from contact with the bottom which might accumulate moisture (hence the drilled holes on the blue plastic to act as drain). The gray 4-pair UTP cable can be seen on the lower left.